Depending on the area of use and application, ammeters have pointer instruments on which the current strength is displayed analogously by pointer deflection. These can be field instruments with switchable current ranges or built-in instruments. In addition, there are digitally displaying ammeters with range switching for direct and alternating current. These ammeters display the measured value on a display module, which usually uses LCD technology.
Since the circuit is broken when measuring current with an ammeter and the ammeter is inserted into the circuit, the entire current flows through the measuring device. To keep the measuring error caused by the inserted ammeter as low as possible, the internal resistance should be extremely low. It is in series with the load and causes a voltage drop due to the current flow, which has the effect of a measurement error. Current measurements without breaking the circuit are made with current clamps.
The most sensitive current measuring range is available when only the internal resistance of the measuring element is in the circuit. Measuring ranges for higher current values are realized by connecting resistors in parallel to the measuring element. These parallel resistors, through which the measuring current is divided, are also called shunt resistors. In some ammeters, range switching is automatic to protect the measuring mechanism. In addition, various ammeters are protected against overload and have reverse polarity protection, i.e. the positive and negative connecting leads can be reversed without damaging the ammeter.
The accuracy of ammeters depends on the reading accuracy of the pointer instrument and whether it is without or with a mirrored scale. For the classification of accuracy, ammeters are divided into classes corresponding to the percentage accuracy of readings.